The present invention relates to trench MOSFET devices, and more particularly to trench MOSFET devices having low source contact resistance.
A trench MOSFET (metal-oxide-semiconductor field-effect transistor) is a transistor in which the channel is formed vertically and the gate is formed in a trench extending between the source and drain. The trench, which is lined with a thin insulator layer such as an oxide layer and filled with a conductor such as polycrystalline silicon (also known as polysilicon), allows less constricted current flow and thereby provides lower values of specific on-resistance. Examples of trench MOSFET transistors are disclosed, for example, in U.S. Pat. Nos. 5,072,266, 5,541,425, 5,866,931 and 6,031,265, the disclosures of which are hereby incorporated by reference.
As a specific example, FIG. 1 illustrates half of a hexagonally shaped trench MOSFET structure 21, which is disclosed in U.S. Pat. No. 5,072,266. The structure includes an n+ substrate 23, upon which is grown a lightly doped n epitaxial layer 25 of a predetermined depth depi. Within the epitaxial layer 25, p body region 27 (p, p+) is provided. In the design shown, the p body region 27 is substantially planar (except in a central region) and typically lays a distance dmin below the top surface of the epitaxial layer. Another layer 28 (n+) overlying most of the p body region 27 serves as source for the device. A series of hexagonally shaped trenches 29 are provided in the epitaxial layer, opening toward the top and having a predetermined depth dtr. The trenches 29 are typically lined with oxide and filled with conductive polycrystalline silicon, forming the gate for the MOSFET device. The trenches 29 define cell regions 31 that are also hexagonally shaped in horizontal cross-section.
A typical MOSFET device includes numerous individual MOSFET cells that are fabricated in parallel within a single chip (i.e., a section of a semiconductor wafer). Hence, the chip shown in FIG. 1 contains numerous hexagonal-shaped cells 31 (portions of five of these cells are illustrated). Cell configurations other than hexagonal configurations are commonly used, including square-shaped configurations. In a design like that shown in FIG. 1, the substrate region 23 acts as a common drain contact for all of the individual MOSFET cells 31. Although not illustrated, all the sources for the MOSFET cells 31 are typically shorted together via a metal source contact that is disposed on top of the n+ source regions 28. An insulating region, such as borophosphosilicate glass (not shown), is typically placed between the polycrystalline silicon in the trenches 29 and the metal source contact to prevent the gate regions from being shorted with the source regions. Consequently, to make gate contact, the polycrystalline silicon within the trenches 29 is typically extended into a termination region beyond the MOSFET cells 31, where a metal gate contact is provided on the polycrystalline silicon. Since the polycrystalline silicon gate regions are interconnected with one another via the trenches, this arrangement provides a single gate contact for all the gate regions of the device. As a result of this scheme, even though the chip contains a matrix of individual transistor cells 31, these cells 31 behave as a single large transistor.
It has been found that, as the sheet resistance over the p-body increases, the voltage drop across the p-body also increases, making the parasitic NPN transistor more susceptible to being incidentally turned on. For example, during avalanche breakdown, the parasitic transistor can be activated incidentally, which can seriously degrade the overall performance of the device and can even cause permanent damage to the device.
One approach by which the resistance of the body region (and hence the voltage drop across the body region) can be decreased in a trench MOSFET device is illustrated in FIG. 1. Within the cell region 31, a p+ portion of the body region 27 rises to the top surface of the epitaxial layer and forms an exposed pattern 33 in a horizontal cross section at the top surface of the cell region 31. This p+ central portion has a higher p-dopant concentration than the portion of the p-body region 27 that is adjacent the channel. This lowers the parasitic resistance of the P-body region 27, improving the robustness of the transistor cell. This is achieved because the voltage drop across the body regions 27 of the device is reduced, likewise reducing the parasitic resistance and hence reducing the likelihood of incidentally turning on the parasitic NPN transistors.
(It is also noted that, for the specific design illustrated in FIG. 1, the p+ central portion of the p body region 27 extends to a depth dmax below the surface of the epitaxial layer that is greater than the trench depth dtr for the transistor cell so that breakdown voltage tends to occur away from the trench surface and in the bulk of the semiconductor material.)
A similar approach through which the resistance of the body region can be decreased in a trench MOSFET device is described in U.S. Pat. No. 6,031,265. FIG. 2 is taken from this patent and illustrates a portion of a trench MOSFET in which an N+ substrate 105 supports an N epi-layer 110. Each transistor cell of this device includes a trenched gate 125, an N+ source region 140, and a P-body region 130. An insulation layer 145 is also provided as is typical. Each transistor cell further includes a deep P+ region 138 formed in the P-body region. The deep P+ region 138 has a higher P-dopant concentration than the surrounding P-body, lowering the parasitic resistance of the P− body region 130 and improving the robustness of the transistor cell. A shallow P+ region 139 is further provided in the body region 130 to reduce the contact resistance at the metal contact 170.